Dynamical evolution of CO2 and H2O on garnet electrolyte elucidated by ambient pressure X-ray spectroscopies

Garnet-type Li6.5La3Zr1.5Ta0.5O12 (LLZO) is considered a promising solid electrolyte, but the surface degradation in air hinders its application for all-solid-state battery. Recent studies have mainly focused on the final products of the LLZO surface reactions due to lacking of powerful in situ characterization methods. Here, we use ambient pressure X-ray spectroscopies to in situ investigate the dynamical evolution of LLZO surface in different gas environments. The newly developed ambient pressure mapping of resonant Auger spectroscopy clearly distinguishes the lithium containing species, including LiOH, Li2O, Li2CO3 and lattice oxygen. The reaction of CO2 with LLZO to form Li2CO3 is found to be a thermodynamically favored self-limiting reaction. On the contrary, the reaction of H2O with LLZO lags behind that of CO2, but intensifies at high pressure. More interestingly, the results provide direct spectroscopic evidence for the existence of Li+/H+ exchange and reveal the importance of the initial layer formed on clean electrolyte surface in determining their air stability. This work demonstrates that the newly developed in situ technologies pave a new way to investigate the oxygen evolution and surface degradation mechanism in energy materials.

The increasing desire for higher capacity and better safety energy storage technology is fostering the revolution of lithium-ion batteries (LIBs) [1][2][3] .Among all possible anode materials, Li metal is considered as the ultimate choice to boost the energy density in LIBs because of its high theoretical capacity (3869 mA h g −1 ) and low redox potential 1,4 .However, the growth of Li dendrites and the severe volume changes limit the applications of Li metal batteries [5][6][7] .Non-flammable solidstate electrolytes (SSEs) with high shear modulus and Li + transference number have potential to solve these problems 8,9 .Among various types of SSEs, garnet-based solid electrolyte Li 7 La 3 Zr 2 O 12 is highly promising due to its high ionic conductivity (up to 1 mS cm −1 at 25 °C), high electrochemical stability towards Li metal, the feasibility of mass production and possible storage in air 10,11 .Nevertheless, previous findings have pointed out that a lithiophobic Li 2 CO 3 layer forms on the surface of LLZO as a result of exposure to CO 2 and H 2 O in air 12,13 .This thin Li 2 CO 3 layer, moreover, have shown to surprisingly governs the interface property, leading to poor interfacial contact and high interfacial impedance between LLZO and electrode materials 14 .
Different pathways for the surface reaction have been proposed to describe the actual reaction between LLZO and air.The density functional theory (DFT) calculations indicate that, thermodynamically, CO 2 can react directly with LLZO to produce Li 2 CO 3   13   .However, the reaction seems kinetically slow as only a negligible amount of Li 2 CO 3 was observed experimentally on the surface of LLZO after long-term exposure to anhydrous air 15 .In another highly probable pathway, H 2 O first reacts with LLZO to form LiOH through Li + /H + exchange, then a substantial part of LiOH transforms into Li 2 CO 3 after exposure to CO 2 16   .Spontaneous Li + /H + ion exchange is believed not to change the cubic garnet structure of LLZO, but blocks Li + transmission channel 16,17 .Therefore, a core−shell structure is proposed, comprising a garnet core surrounded by a proton-rich garnet shell and a LiOH/Li 2 CO 3 outer layer.Up to now, this core-shell structure has only been studied by destructive characterization techniques such as argon ion sputter etching 18 , and the reaction mechanism of Li 2 CO 3 formation has not been fully understood.Thus, it is of great significance to develop in situ techniques with depth-profiling capability to comprehensively probe the initial reactions at gas/solid interface, which can help us understand the different thermodynamic and kinetic processes for CO 2 and H 2 O on LLZO surface, and eventually provide guidance for avoiding the formation of Li 2 CO 3 and building an excellent interface.
Synchrotron-based core-level X-ray spectroscopies, including X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectrum (XAS), have been widely used to monitor the gas/solid interface in catalysis and environmental science [19][20][21] .Recent developed in situ spectroscopy techniques can be utilized to investigate the reaction between gas and the solid surface with elemental and chemical sensitivities.Additionally, for soft-X-rays, XAS in total-electronic-yield (TEY) mode can support a detection depth of ~10 nm, while signals in auger-electron-yield (AEY) mode usually come from a depth of ~3 nm 22 .Thus, simultaneous detection of XAS in both TEY and AEY mode along with XPS allows us to obtain an in-depth analysis of the chemical evolution on the surface and sub-surface.However, due to the poor air stability and very similar spectroscopic fingerprints, lithium containing species such as Li 2 O, Li 2 O 2 , LiOH, Li 2 CO 3 and LLZO are very difficult to identify, thus very limited studies have combined all these surface methods in ambient pressure for energy materials characterization until now.
In this paper, the initial surface chemistry and evolution mechanism of LLZO in H 2 O and CO 2 were in situ investigated by ambient pressure X-ray photoelectron and absorption spectroscopy.In particular, the newly developed ambient pressure mapping of resonant Auger spectroscopy (AP-mRAS) method clearly identifies lattice oxygen (LLZO) and various surface oxygen species such as LiOH, Li 2 O and Li 2 CO 3 , and the in situ depth-profiling technology can deduce the flow direction of lithium.We find that CO 2 reacts directly with LLZO thermodynamically, but the reaction is limited by surface active sites and a lack of oxygen supply.The reaction of H 2 O with LLZO needs a relatively higher pressure than CO 2 through Li + /H + exchange.However, the reaction is more intense and continuous, forming LiOH.Comparing LLZO with Li 1.5 Al 0.5 Ge 1.5 P 3 O 12 (LAGP), we found that the characteristics of the interface layer initially formed on the surface and the stability of the material structure may be the decisive factors for its resistance to air degradation.

Clean LLZO surface obtained by low temperature treatment
The LLZO pellet suffers severe surface degradation after exposure to air. Figure 1a, b shows the surface and cross-section morphology of the LLZO pellet after exposure to air for 2 months.Large amounts of amorphous structures are visible around the voids of LLZO pellet, assignable to Li 2 CO 3 which is evidenced by the presence of C and O in energy-dispersive X-ray spectroscopy (EDS) analysis in Fig. 1c 23,24 .After polishing by sandpaper and wiping by alcohol, the LLZO particles can be clearly observed on the surface with a small amount of residual debris Fig. 1d, e.The results also indicate that completely clean LLZO surface cannot be obtained only by physical polishing.The X-ray diffraction (XRD) pattern of the aged LLZO pellet is displayed in Fig. 1f.The diffraction peaks match well with the cubic garnet phase with no other impurities, which means that the phase of the bulk LLZO is barely changed during the surface chemical evolution in air, and the Li 2 CO 3 contamination may have an amorphous structure.
The schematic diagram of the entire in situ experimental processes in this work are shown in Fig. 2. A clean LLZO surface is necessary for monitoring of the degradation mechanism.To achieve a clean LLZO surface, the polished LLZO pellet was annealed at 350 °C in vacuum for 30 min and then annealed in 1 × 10 −6 mbar O 2 at 350 °C for 30 min, as shown in Fig. 2a-e.During the vacuum annealing process, in the range of 30 to 350 °C under ultra-high vacuum (UHV) condition, XPS, mass spectrum and XAS in AEY and TEY mode were carried out to characterize the surface chemical evolution.The schematic and the photo of the instrument are shown in Fig. 3a, b.XPS and XAS in AEY mode were conducted by the Scienta Hipp3 analyzer, while the XAS in TEY mode was measured by a pico-ammeter 25,26 .
The polished LLZO pellet was mounted onto a sample holder under ambient air (which took about 15 min) and were pumped into the instrument.Thus, a strong signal of Li 2 CO 3 and weak signals of La and Zr could be observed in XPS spectra as shown in Fig. 3c and Supplementary Fig. 1.The peak at 284.6 eV in C 1 s spectrum can be assigned to the C-C (sp2 ) bond of the surface contamination 27 .The O K edge AEY and TEY spectra in Supplementary Fig. 2 are consistent with our XPS results that the surface of LLZO is mainly Li 2 CO 3 before annealing.A small signal of LLZO is present at ~531.9 eV in the AEY spectrum, which is slightly weaker than in the TEY spectrum with a larger investigation depth of ~10 nm, indicating that the thickness of the contamination layer is several nanometers.
In order to determine whether the surface Li 2 CO 3 can be completely removed by vacuum annealing, the O 1 s and C 1 s XPS spectra were measured at 350 °C.Only the peak of LLZO at around 529 eV labeled as O(lattice) can be detected in the O 1 s XPS spectrum as shown in Supplementary Fig. 3a.The results indicate that a clean LLZO surface without other O containing species can be achieved by lowtemperature vacuum annealing 28 .However, after cooling down the LLZO pellet to room temperature, the peak of Li 2 CO 3 appears again, as evident from Fig. 3c, indicating the clean LLZO surface may be very sensitive to residual CO 2 29 and can form Li 2 CO 3 on the surface even at a pressure below 1 × 10 −8 mbar.Li 2 CO 3 is difficult to be observed in the AEY and TEY spectra of annealed LLZO at RT in Supplementary Fig. 2, indicating that very little Li 2 CO 3 is generated on the surface during the cooling process.Thus, a clean surface of LLZO with very small amount of Li 2 CO 3 is obtained by vacuum annealing.
Further, we systematically investigated the influence of temperature rise on the ratios of O(lattice)/O(CO 3 )/C(C-C), delineated in Fig. 3d.The decomposition temperature of the surface Li 2 CO 3 can be identified to be around 300 °C, which is much lower than the reported reaction temperature (over 620 °C for the reaction Li 2 CO 3 → Li 2 O + CO 2 ) 30,31 .No signal of Li 2 O was detected in the O 1 s XPS and O K edge mRAS data of annealed LLZO as shown in Supplementary Fig. 4. To probe the decomposition reaction at low-temperature, the decomposition gaseous products were detected by in situ mass spectrum, and the results are exhibited in Fig. 3e.Both CO 2 and H 2 O were simultaneously observed as the decomposition products around 300 °C.Thus, the actual reaction at the LLZO surface during the vacuum annealing process could be: xLi 2 CO 3 + Li 6.5- O 2 at 350 °C for 30 min.After annealing, the signals of other elements remained basically unchanged, and the signal of contaminated carbon species significantly decreased.

Clear identification of lithium containing species using AP-mRAS and XAS
Due to the poor air stability and very similar spectroscopic fingerprints, lithium containing species are very difficult to identify by surface sensitive characterization methods 33,34 .Here, using the newly developed AP-mRAS method, we can carefully identify lattice and surface oxygen in lithium containing species before studying the reaction mechanism.The mechanism and characteristics of mRAS compared with mapping of resonant inelastic X-ray scattering (mRIXS) are shown in Supplementary Fig. 5, the incident photon energy is scanned across the absorption edge, and the emitted Auger electrons at each resonant energy are further resolved in kinetic energy (KE).
Li metal was in situ scraped using a wobble-stick with sharp blade as shown in Supplementary Fig. surface shows an oblique spot at photon energy around 531.9 eV.The inclination of the spot depends on the d orbital property that hybridized with the O 2p orbital.Furthermore, clean LLZO surface shows a localized feature in KE direction with a much smaller photon energy which is quite different from Li 2 O, which confirms the surface contamination is fully wiped out.Two parallel structures with the same binding energy are Ta 4 f peaks, which come from the doped Ta in LLZO.For the Ap-mRAS of LLZO after the introduction of 1 mbar CO 2 , the coexistence of LLZO and Li 2 CO 3 features are observed, which proves that only a small amount of Li 2 CO 3 (<3 nm) is produced on the surface.Compared with the signal of Li 2 CO 3 , the signal of LiOH is much stronger after 0.5 mbar H 2 O is introduced.LiOH also displays a vertical symmetrical feature at photon energy of 532.8 eV and its intensity extends along the KE direction.Li 2 CO 3 and LiOH show very close KE values, which is 1.5 eV lower than that of LLZO as shown in Fig. 4d.These results reveal that the inclination, localization and the KE position of the intensity center in the two-dimensional spectrum can be used to accurately identify lattice oxygen (LLZO) and surface oxygen in lithium containing species such as LiOH, Li 2 O and Li 2 CO 3 .Combined with in situ mass spectrometry, AP-mRAS is a potential method to study anionic redox behavior in various cathode and catalytic materials.After the identification of lithium containing species, we can use the in situ ambient pressure technologies to study the specific reaction kinetics and thermodynamic process, which will be detailed in the following.

The detailed reaction process of clean LLZO surface with CO 2
The advanced synchrotron radiation ambient pressure technology helped us to slow down the rapid reactions of CO 2 /H 2 O on clean LLZO surface, so that we can clearly observe the thermodynamic and kinetic reaction processes.The kinetics of the reaction of clean LLZO with CO 2 was investigated by in situ ambient pressure APXPS and XAS measurements to achieve a depth-profiling analysis of the surface reaction.Figure 5a, b shows the O 1 s and C 1 s APXPS spectra of the LLZO at increasing pressure of CO 2 .During the reaction, the reaction product is almost pure Li 2 CO 3 which is illustrated in Supplementary Fig. 6.The peak intensity of CO 3 2-at around 531.5 eV enhances with the increase of CO 2 pressures from 1 × 10 −6 -1 × 10 −2 mbar, implying that CO 2 can rapidly react with LLZO to form Li 2 CO 3 even at low CO 2 pressure.However, the peak intensity of CO 3 2-remains almost unchanged upon increasing the pressure of CO 2 from 1×10 −2 to 1 mbar.The results reveal that the reaction of CO 2 on the LLZO surface may be restricted by surface active sites or oxygen supply from the sub-layer.
The O K edge AEY and AP-mRAS spectra of the LLZO pellet at increasing CO 2 pressures are displayed in Fig. 5c and Supplementary Fig. 7.The variation in the intensity of the Li 2 CO 3 peak is consistent with that of XPS; the peak increases from 1 × 10 −6 to 1 × 10 −2 mbar and then stabilizes from 1 × 10 −2 to 1 mbar.We normalized the AEY data at different CO 2 pressures to achieve a clearer internal reaction image and then subtracted the UHV data (I pressures -I UHV ).The difference results are presented in Fig. 5d, where the spectrum at 1 × 10 −6 mbar The variations of Li 1 s, Zr 3d, and La 4d APXPS spectra at increasing CO 2 pressure are shown in Supplementary Fig. 8.The Li 1 s peak shifts to higher binding energy as the CO 2 pressure rises, indicating the formation of Li 2 CO 3 on the surface.No noticeable spectral line shape changes are seen in Zr 3d and La 4d spectra.Thus, the possible reaction between CO 2 and clean LLZO surface can be described as following: the CO 2 molecules may easily adsorb on particular oxygen sites on the surface of LLZO at very low CO 2 pressure.Subsequently, the lithium in the sub-layer are pulled to the surface to form Li 2 CO 3 through the reaction: Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 + xCO 2 → Li 6.5−2x La 3 Zr 1.5 Ta 0.5 O 12-x + xLi 2 CO 3 .In contrast, LLZO is not a good oxygen ionic conductor at room temperature, the oxygen in the sublayer have difficulty migrating to the surface.When the above definite oxygen sites are fully occupied, CO 2 cannot react with LLZO further, resulting in the Li 2 CO 3 layer being only 1-3 nm thick.Consequently, the CO 2 reaction pathway is considered kinetically slow which is coincided with the negligible amount of Li 2 CO 3 formed on garnet pellets after exposure to dry air 18,38,39 .

The detailed reaction process of clean LLZO surface with H 2 O
After the CO 2 experiment, the LLZO pellet was treated using the same annealing method as shown in Fig. 2f-h.Interestingly, the Li 2 CO 3 could not be removed completely by vacuum annealing as the signal of Li )/O(lattice) and O(OH -)/O(lattice) during the introduction of CO 2 and of H 2 O, respectively, and the corresponding results can be viewed in Supplementary Fig. 10.The reaction with CO 2 shows a deceleration process, while that of H 2 O accelerates as the pressure increases.These results corroborate that the reaction of CO 2 is a thermodynamically favorable route compared with that of H 2 O at low pressure.Next, we introduced a mixture gas of 0.5 mbar H 2 O and 0.5 mbar CO 2 , most of the surface LiOH is converted into Li 2 CO 3 as shown in Fig. 6a and Supplementary Table 1.
The O K edge AEY and AP-mRAS spectra, shown in Fig. 6c and Supplementary Fig. 11, give a detailed analysis of the surface reaction, and the obtained results are consistent with the APXPS.The intensity of Li 2 CO 3 enhances as the pressure of H 2 O increases from UHV to 1 × 10 −4 mbar.However, the signal of LiOH at 532.8 eV nearly covers the signal of LLZO over 0.5 mbar, indicating that the reaction of H 2 O is more severe than CO 2 .Since no effective TEY spectra could be collected at the 0.5 mbar H 2 O pressure in Fig. 6e, we collected the spectrum when returning from 0.5 mbar to UHV instead.Even though the peak of LiOH is significantly intense, the LLZO signal can still be detected in the TEY spectrum at UHV, indicating that the thickness of LiOH layer at 0.5 mbar H 2 O is slightly <10 nm, much thicker than the Li 2 CO 3 layer formed at the pressure of 1 mbar CO 2 .Notably, signals of O (2-x)-and O (2-x+y)-at around 530.8 eV were absent in all of the AEY and TEY difference spectra shown in Fig. 6d, f, signifying no valence change of oxygen in LLZO sub-layer after exposure to H 2 O.These results establish the existence of Li + /H + exchange, i.e., the H + fill the vacancies after the Li + are pulled to the surface.After introducing a mixture gas of 0.5 mbar H 2 O and 0.5 mbar CO 2 , the peak of Li 2 CO 3 in both AEY and TEY spectra is unambiguously observed, indicating the existence of the surface reaction: 2LiOH + CO 2 → Li 2 CO 3 + H 2 O.

Comparison of LLZO with LAGP
In order to find the possible determining factor for the air stability of electrolyte materials, we used the same in situ methods to study LAGP, which is an air stable solid electrolyte.The LAGP ceramic pellet was removed from its sealed packaging and placed directly into the test chamber from air without polishing.The signals of P, Ge and O can still be seen on the surface, indicating that LAGP has much better air stability than LLZO.Vacuum annealing at 350 °C hardly changes the composition of the surface.Then, we annealed the sample in 1 × 10 −6 mbar O 2 at 350 °C for 30 min.As shown in the Supplementary Fig. 12, it can be seen that the surface C of the sample at 350 °C significantly decreases, while the signals of O and P are greatly enhanced.However, when the sample was cooled to room temperature, the signal of C increased again while the signal of O and P decreased.No such carbonization phenomenon is observed on the surface of LLZO.
We conducted APXPS experiments on clean LAGP and the results are shown in Supplementary Fig. 13.LAGP cannot react with CO 2 and H 2 O even at a high pressure of 0.5 mbar.By comparing two electrolyte materials, we infer that the stability of LAGP may come from the hydrophobic carbon layer generated on the surface and the stability of PO 4 structure 42 .Due to the stability of surface O, it is difficult for LAGP to directly react with CO 2 to generate Li 2 CO 3 .On the contrary, LLZO surface can react directly with CO 2 to form a Li 2 CO 3 , which is a hydrophilic layer, may cause the occurrence of subsequent severe reactions.The results indicate that the hydrophilicity and hydrophobicity of the initial layer formed on the clean electrolyte surface, as well as the stability of the surface O structure, may play an important role in determining the air stability of solid electrolyte materials.

Discussion
The human factor in peak fitting has always been a problem in XPS data processing, especially in the overlapped O 1 s XPS data.Thus, the mRAS and AEY method are developed to assist XPS in species identification.The mRAS and AEY method can be implemented at any synchrotron radiation XPS end-station without requiring additional hardware.The mRAS and AEY has a detection depth slightly higher than XPS which not only can assist XPS in species identification, but also provide deep analysis abilities.In addition, its identification of species is also clearer and more intuitive because additional dimensions can result in more fingerprint features just like mRIXS as shown in Supplementary Fig. 5. Due to the dimensional improvement, mRIXS in O K edge has played an important role in the study of oxygen redox in cathode materials.The detection efficiency of mRAS is much higher than that of mRIXS, and the detection is also not affected by the gas and electrochemical environment.Thus, the mRAS method combined with near ambient pressure depth-profiling characterization methods possess the substantial potential and, therefore, should be widely popularized to study the air stability and the thermodynamics and kinetics process of gas/solid interface in energy materials.
In conclusion, in situ ambient pressure depth-profiling techniques were initiatively used to elucidate the dynamical evolution of LLZO pellet with CO 2 and H 2 O. Low-temperature vacuum annealing helped us to obtain a clean LLZO surface through the reaction: xLi 2 CO 3 + Li 6.5-2x H 2x La 3 Zr 1.5 Ta 0.5 O 12 → Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 + xH 2 O + xCO 2 , where the H travel from LHLZO on the surface of LLZO.The APXPS, AP-mRAS and APXAS results experimentally prove that the reaction of CO 2 with LLZO to form Li 2 CO 3 is a thermodynamically favored path.However, the CO 2 reaction is restricted by the hindered oxygen supply from the sub-layer, affording only 1-3 nm thick Li 2 CO 3 layer.The driving force of the reaction leads to the formation of a lithium gradient in the sublayer of LLZO.Moreover, Li + /H + exchange was directly observed as no lithium gradient appeared during the reaction.This exchange is more intense when H 2 O reaches a higher pressure.Our results give a precise mechanism of the initial reaction of LLZO with CO 2 and H 2 O and reveal the initial layer may play an important role in determining the air stability of solid electrolyte materials.

Materials and characterizations
The starting materials of Li 2 CO 3 (Alfa Aesar, 99.9%), La 2 O 3 (Alfa Aesar, 99.9%), ZrO 2 (Alfa Aesar, 99.5%) and Ta 2 O 5 (Aladdin, 99.5%) were mixed in stoichiometric amounts with 15 mol% Li 2 CO 3 in excess.The mixture was ball-milled in 2-propanol for 12 h with agate balls in an agate vial, and then dried and heated in air at 1150 °C for 12 h.Then the ball-milling was repeated once, and the powder was sieved with a mesh number of 600 to obtain fine particles.The pellets were made by hotpressing of the as-prepared LLZO powder in a flowing argon atmosphere at a temperature of 1050 °C under a constant pressure of 50 MPa for 1 h.The size of pellet for the operando experiment was 0.8 mm in thickness and 12 mm in diameter 23 .0.3 mm thick LAGP ceramic pellet was purchased from Hefei Kejing Material Technology Co., Ltd.The X-ray powder diffraction (XRD) was performed using a Bruker D8 advance with Cu Kα radiation (λ = 1.54178Å).Sintered and densified LLZO pellet was carried out from the glove box, then dry polished in air progressively using polishing paper with grit number from 800 to 1500, and the surface was wiped by alcohol.After polishing treatment, the polished and aged pellets were quickly transferred to the test chamber for the surface and cross section scanning electron microscopy (SEM) images, using a JEOL JSM-7800F field emission microscope.

Operando ambient pressure experiment
In situ annealing, ambient pressure mapping of resonant Auger electronic spectroscopy (AP-mRAS), ambient pressure X-ray photoelectron spectroscopy (APXPS) and X-ray absorption spectroscopy (APXAS) were carried out at BL02B at the Shanghai Synchrotron Radiation Facility (SSRF).The pellets were brought to the SSRF through an aluminum plastic bag sealed in a glove box.Before transferred to UHV chamber, the LLZO pellet was polished in air by sandpapers with grit number from 800 -1500 to achieve parallel faces.The polishing thickness was sufficient to ensure that the surface contamination layer is completely polished off and then the surface was wiped by alcohol.The polished pellet was mounted onto a sample holder under ambient air (which took about 15 min) and were pumped into the instrument.LAGP and Li metal were pumped into the chamber without polishing process.Li metal was in situ scraped using a wobblestick with sharp blade surface.For the vacuum annealing process, an infrared laser heater (912 nm wavelength, PREVAC) was used for heating from backside of the sample holder.The temperature was monitored by the K type thermocouple attached onto the LLZO pellet.The temperature was increased at the rate of 3 °C/min from room temperature to 370 °C and then kept at 350 °C for 30 min before spectroscopic measurements.The mass spectrum was collected by using MKS e-Vision 2 residual gas analyzer (RGA) installed on the analysis chamber.After conducting high-temperature measurements, LLZO pellet was naturally cooled to room temperature at a base pressure ~1 × 10 −8 mbar for subsequent in situ characterization.After vacuum annealing, the LLZO pellet was annealed in 1×10 −6 mbar O 2 at 350 °C to reduce surface contaminated carbon.
The beamline 02B was a bending magnet beamline providing photons with energy range from 40 -2000 eV.The photon flux was about 10 11 photons/s and the energy resolving power was up to 13000.The C 1 s, O 1 s, Li 1 s, Zr 3d and La 3d XPS spectra were all collected at the photon energy of 650 eV with a step size of 0.1 eV using the Hipp-3 electron energy analyzer (Scienta Omicron).The photon energy was calibrated by a gold foil on the sample holder and the binding energy was calibrated by the C 1 s peak on LLZO at 284.6 eV.The APXAS data of O K-edge were collected simultaneously by using Auger electron yield (AEY) mode with investigation depth ~3 nm and total electron yield (TEY) mode with the penetration depth ~10 nm.The photon energy step was set at 0.2 eV for O K-edge mRAS and TEY experiments and the collection time of each mRAS mapping was about 12 min.The window of kinetic energy for O K-edge mRAS was set to 512 ±7 eV.All the spectra have been normalized to the beam flux measured by the upstream gold mesh.The stainless-steel tubes for CO 2 (99.999%) and water vapor were baked in vacuum conditions and then flushed several times using high-purity CO 2 and water vapor, respectively, before introducing the gases into the chamber.High-purity CO 2 and water vapor were introduced into the reaction chamber by controlling two independent all-metal leak valves (VACGEN) as shown in Fig. 3a.The gas pressure was read by a capacitance film vacuum gauge (PFEIFFER CMR 363) attached on the chamber.All spectra were collected after the pressure had stabilized for 15 min.For the experiments of a mixture of H 2 O + CO 2 , the water vapor of 0.5 mbar was firstly maintained, then CO 2 was introduced until the total pressure stabilized at 1 mbar.

4 .Fig. 1 |Fig. 2 |
Fig. 1 | Surface degradation of LLZO after exposure to air. a Surface and b cross-section scanning electron microscope (SEM) images of aged LLZO pellet.c EDS elemental mappings of the aged sample.d Surface and e cross-section SEM images of polished LLZO pellet.f XRD patterns of the aged LLZO pellet.

Fig. 3 |Fig. 4 |Fig. 5 |
Fig. 3 | Clean LLZO surface obtained by low temperature vacuum annealing.a Schematic and b real instrument for the operando ambient pressure experiment.c XPS spectra of the LLZO pellet before and after vacuum annealing at room temperature.d The influence of temperature rise on the ratios of O(lattice)/ O(CO 3 2-) and C(CO 3 2-)/C(C-C).e In situ mass spectrum results in the temperature 2 CO 3 was still observable in the O 1 s XPS spectrum at 350 °C in Supplementary Fig. 9a.The results confirm the following decomposition reaction of Li 2 CO 3 at 350 °C: xLi 2 CO 3 + Li 6.5-2x H 2x La 3 Zr 1.5 Ta 0.5 O 12 → Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 + xH 2 O + xCO 2 .Thus, the decomposition reaction of Li 2 CO 3 would require a higher temperature if the hydrogens in the sub-layer LLZO are fully removed.Furthermore, the reaction of H 2 O with the LLZO pellet was studied by APXPS and APXAS.The O 1 s XPS spectra and the product evolution diagram at increasing H 2 O pressure are displayed in Fig.6a, b. Surprisingly, the peak intensity of CO 3 2-at 531.5 eV enhances with increasing H 2 O pressure from UHV to 1 × 10 −4 mbar, probably due to the gas path can only be cleaned to 1 × 10 −7 mbar and there is a small amount of residual CO 2 .The results also indicate that the reaction of Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 + xCO 2 → Li 6.5−2x La 3 Zr 1.5 Ta 0.5 O 12-x + xLi 2 CO 3 at clean LLZO surface may be a thermodynamically favorable route compared to the reaction of LLZO with H 2 O, which is consistent with a just published article by Grey's group40 .As the pressure of H 2 O increases to 0.5 mbar, the signal of OH -at 530.9 eV 41 almost covers the peak of CO 3 2-, revealing that H 2 O mostly reacts with LLZO at 0.5 mbar and the reaction products are much more than that of CO 2 .Moreover, we compared the variation of O(CO 3 2-